The present invention relates to internal-combustion engines of the type comprising:
at least one induction valve and at least one exhaust valve for each cylinder, each valve being provided with respective elastic means that bring back the valve into the closed position to control communication between the respective induction and exhaust ducts and the combustion chamber;
a camshaft for operating the induction and exhaust valves of the cylinders of the engine by means of respective tappets, each induction valve and each exhaust valve being actuated by a cam of said camshaft;
in which at least one of said tappets controls the respective induction or exhaust valve against the action of said elastic return means via the interposition of hydraulic means including a hydraulic chamber containing fluid under pressure;
said hydraulic chamber containing fluid under pressure being connectable, via a solenoid valve, to an outlet channel for decoupling the valve from the respective tappet and causing fast closing of the valve under the action of respective elastic return means;
said hydraulic means further comprising a piston associated to the stem of the valve and slidably mounted in a guide bushing, said piston being set facing a variable-volume chamber defined by the piston inside the guide bushing, said variable-volume chamber being in communication with the hydraulic chamber containing fluid under pressure by means of an end aperture of said guide bushing, said piston having an end appendage designed to be inserted into said end aperture during the final stretch of the closing stroke of the valve in order to restrict the communication port between said variable-volume chamber and said hydraulic chamber containing fluid under pressure, so as to slow down the stroke of the valve in the proximity of its closing,
in which the aforesaid outlet channel communicates with an accumulator for fluid under pressure and with a feed pipe for feeding the fluid coming from a feed pump.
An engine of the type referred to above is, for example, described and illustrated in the European patent applications Nos. EP-A-0 803 642 and EP-A-1 091 097 filed by the present applicant.
Studies and tests carried out by the present applicant have shown that some problems may arise during operation, particularly when the engine stops running at low temperatures on account of the consequent variations in the volume of the hydraulic fluid (typically oil). When the engine has not been running for a long time in a low-temperature environment, the oil in the low-pressure circuit, i.e., in the section between oil feed and the solenoid valve, contracts and leaks, so freeing spaces in the circuit which generate air bubbles that are difficult to eliminate and subsequently impair operation of the system during engine starting.
The purpose of the present invention is to overcome the above-mentioned problem by providing a system which reduces as far as possible formation of air bubbles in the circuit following upon variations in the volume of the hydraulic fluid resulting from variations in the temperature of the fluid when the engine is turned off and from leakage of the hydraulic fluid through the gaps resulting from constructional play of the various components.
With a view to achieving this purpose, the subject of the invention is an engine having all the characteristics referred to at the beginning of the present description and moreover characterized in that connected to the aforesaid channel, upstream of the solenoid valve, is at least one supplementary fluid reservoir, bled off to the atmosphere, which is partially occupied by the fluid in the normal operating condition of the engine and which is emptied partially of fluid when the engine stops running at low temperatures, and which fills up, instead, in the event of expansion of the hydraulic fluid resulting from an increase in temperature.
In other words, the system is equipped with a sort of expansion box or expansion vessel which contains a certain amount of hydraulic fluid and which is consequently able to return this fluid to the circuit in the low-temperature condition so as to prevent formation of air bubbles in the circuit, and is able to receive the fluid back into it again when the temperature rises.
In one first embodiment, the aforesaid supplementary reservoir consists of a vessel distinct from the accumulator and has a bottom end connected to the circuit and a top end bled off to the atmosphere.
In another embodiment, the supplementary reservoir consists of the same vessel as the accumulator, which in this latter case has a piston with a restricted hole having a pre-determined diameter which enables the expanding fluid to occupy the volume of the accumulator above the piston by passing through said hole. Of course, this solution may be utilized either as an alternative or in addition to the one already mentioned above, which envisages a distinct supplementary reservoir.
According to the invention, it is also possible to contemplate the use, as supplementary reservoir, of the vessel of a pressurizer device that can be associated to the hydraulic circuit according to a technique known from the patent EP-B-0931912 held by the present applicant. This device is provided for the purpose of supplying a piston loaded by a spring with the oil under pressure that circulates in the circuit during operation of the engine, so as to be able to exploit the energy thus accumulated upon starting of the engine after the engine has not been running, in order to guarantee a prompt filling of the hydraulic circuit and a fast response of the system. In the case where such a device is provided, it is possible to envisage also for the latter an arrangement similar to the one described above with reference to the hydraulic accumulator, with an air bleeder to the atmosphere and a restricted hole of a pre-determined diameter in the piston of the device, which enables the expanding oil to flow through said hole into the cavity above the piston.